The present invention relates to combustion of hydrocarbonaceous fuel such as coal.
Under the dual pressures of economic deregulation and tightening environmental regulation operators of combustion systems face the difficult task of increasing system efficiency to increase productivity, while simultaneously reducing pollutant emissions. In many combustion systems, such as coal or oil-fired utility boilers, enhancing productivity comes at the expense of increased pollutant emissions, or vice versa.
One of the few areas of boiler or furnace operation that can have a negative impact on both pollution control and productivity is the distribution of fuel and air to the individual burners. It is well known that most boilers or furnaces have significant variations in the amount of either air or fuel, or both, fed to individual burners in a multi-burner array. This variation in air and fuel flow leads to significant stoichiometric ratio variations among the burners, which in turn reduces combustion efficiency and increases pollutant formation. Burners are typically designed to operate within a specific range of stoichiometric ratios to provide a reasonable compromise between good combustion efficiency and low pollutant formation. For example, in a boiler fired with coal, if the air to fuel ratio is too high, the burner operates too xe2x80x9cleanxe2x80x9d (i.e., fuel-lean) and NOx formation is increased. If the stoichiometric ratio is too low the burner operates too xe2x80x9crichxe2x80x9d (i.e., fuel-rich) and CO and unburned carbon increases.
A wide range of factors can lead to problems with air and fuel distribution. Typical solid fuel fired burners, such as those fired with pulverized coal, consist of two main flows. One flow is the transport air, or primary air, which is used to transport fuel from a common feed location to the individual burners. The other flow is the combustion air, or secondary air, which is often supplied through a common windbox. The combustion air stream, which may be subdivided into multiple air streams in the burner, does not mix with the transport air until the burner outlet. For liquid or gas fired systems the combustion air stream may be the only air fed to the burners, other than the minor amount of compressed air used for atomization.
In most combustion systems air for both streams is supplied through the use of a blower. Typical supply pressures are relatively low, in the range of tens of inches of water column. Therefore, even subtle variations in system construction or design can lead to some burners being starved of air, or of fuel if the transport air is similarly affected. Many burners have register dampers that can be opened or closed to control how much air is fed from the windbox. These dampers may also serve to split the secondary air stream into separate streams according to the burner design. However, the damper design and the tolerances required to allow long term operation of the burners make precise flow control to the burners problematic if not impossible. In coal fired utility boilers it is not uncommon to find that the flow rates of air to some of the burners are off by more than 30% from the design values.
With entrained solid fuels, such as coal, the problem of fuel distribution to the burners becomes even more serious. In the case of pulverized coal transport air passes through the pulverizer, or mill, entrains coal that has been pulverized to the desired size, and carries it to the individual burners. With this type of system not only are there issues related to transport air flow to the individual burners, similar to those discussed above, but the problem is compounded by the need to transport a two-phase fluid without permitting separation of the phases in the pipe. For example, as the coal-laden air stream passes around a sharp bend the coal tends to concentrate in one part of the air stream. This phenomenon, called roping, can lead to poor distribution of fuel to the individual burners. Reduction of air flow in any given leg of the distribution system can also lead to settling of the coal from the transport air stream as the velocities are not adequate to keep the solids entrained. In coal fired utility boilers it is not uncommon to find that the coal flow rates to some of the burners are off by more than 30% from the design values.
In addition to the problems associated with maintaining a uniform coal and airflow distribution, some systems may actually require biasing of either the coal or air to specific burners in the array. For example, when a burner is situated adjacent to a sidewall comprising water-cooled tubes (i.e. steam tubes) the flame temperature of that burner can be significantly reduced by heat transfer to the water. Although this reduced flame temperature can help reduce the formation of thermal NOx, it can lead to increases in CO and unburned carbon, if the burner is operated under rich conditions. Furthermore, corrosion of the waterwall may become an issue. To overcome both these problems it may be necessary to bias the air or coal flow to that particular burner such that the burner operates slightly more fuel lean, which serves to increase the flame temperature and combustion efficiency. Given the difficulties associated with creating a simple uniform coal-air distribution, biasing the burners in this fashion is well beyond current commercial practice.
A number of solutions have been proposed to better control both fuel and air flow. These solutions have demonstrated that significant improvements in pollutant emissions and combustion efficiency can be achieved. However, as discussed in the next section, currently available control techniques tend to be limited in their ability to maintain burner balance.
Numerous means have been proposed to control the distribution of fuel and air to individual burners. One is the inclusion in most burners of dampers to control the secondary airflow to the individual burner. The damper assembly is used to close down the cross sectional area of the flow openings in order to restrict the flow of air through the duct. The design of the dampers tends to make flow control very imprecisexe2x80x94making optimization of the flow extremely difficult.
A number of systems, such as those disclosed in U.S. Pat. Nos. 5,685,240, 5,593,131, 6,293,105 and 5,879,148, have been proposed to control the distribution of fuel to an array of burners. These systems preferentially increase the pressure drop through a given leg of the fuel distribution system and/or the air distribution system to control the flow of fuel or air to that specific burner. These systems have been reasonably successful for those burners firing liquid or gaseous fuels, but have been less so for solid fuels due to problems inherent in the transport of a two-phase fluid. These problems include separation of the phases in the transport line and, particularly for solid fuels, erosion of the devices used to control the flow.
Other prior disclosures differ from the present invention in one or more significant ways. U.S. Pat. No. 5,697,306 discloses a device wherein a stream of air is supplied through a so-called xe2x80x9chollow plugxe2x80x9d. The objective of this device is said to be control of the stoichiometric ratio of the fuel rich portion of a burner. An optimal stoichiometric ratio is disclosed only for this fuel-rich region, based on properties of the fuel. Air is supplied such that it mixes rapidly with the transport air and coal at the exit of the burner to create a mixture with the desired stoichiometric ratio. Even if this invention could be advantageous for controlling the stoichiometric ratio of this fuel rich core, there is no attempt to control the overall stoichiometric ratio of the burner, let alone of an array of burners. Further, by operation of the disclosed device with the addition of a second stream of air based on the coal properties, not on the requirement to balance the burner, operation of this device would quite possibly actually exacerbate the burner to burner unbalance.
U.S. Pat. Nos. 4,903,901 and 5,048,761 describe a system wherein a stream of compressed air is injected into the coal pipe of a burner to control the flow of transport air and coal to that burner. Injecting a stream of compressed air is said to create a recirculation zone within the coal pipe, increasing the pressure drop through the pipe, to limit the flow of coal-laden through the pipe, with an effect similar to that provided by the orifice plates described above. The amount of compressed air supplied, typically up to 1% of the transport air flowrate, is based solely on the need to control flow rather than on any recognition of the need to balance the burners in a multiple-burner furnace.
Most prior art efforts to minimize problems associated with variations in stoichiometric ratio have attempted to create uniform fuel and air flows to each burner in an array of burners. Although these techniques can help to minimize variations, it is extremely difficult to eliminate these variations completely. Thus, there remains a need for an improved method to obtain balance in a plurality of burners, so that each burner operates at a desired optimum of conditions such as the stoichiometric ratio.
One aspect of the present invention is a method for combusting hydrocarbonaceous fuel such as coal in a furnace comprising
(A) providing a furnace which comprises a plurality of burners, means for supplying combustion air to each of said plurality of burners including a common source for the combustion air fed to said plurality of burners, and means for supplying hydrocarbonaceous fuel to each of said plurality of burners, wherein at least one of said plurality of burners is operating at a stoichiometric ratio based on the fuel and combustion air being supplied thereto that is above a predetermined optimum, and
(B) reducing the flow rate of combustion air through said common source to said plurality of burners to the extent that (1) at least one of said plurality of burners is still operating at a stoichiometric ratio, based on the fuel and combustion air being supplied thereto taking into account said reduced flow rate, that is equal to or above said predetermined optimum, and that (2) at least one of said plurality of burners is operating at a stoichiometric ratio, based on the fuel and combustion air being supplied thereto taking into account said reduced flow rate, that is below a predetermined optimum for that burner, and separately feeding gaseous oxidant to at least one of said plurality of burners which is operating at a stoichiometric ratio that is below its predetermined optimum, in an amount of said oxidant such that the stoichiometric ratio of said burner based on the amount of said oxidant and on the reduced flow of combustion air thereto is closer to said predetermined optimum.
In some preferred embodiments, oxidant fed to at least one burner has an oxygen content different from the oxygen content of oxidant fed to any other burner. In other preferred embodiments, the total flow rate of said gaseous oxidant separately fed to at least one burner is 1 to 20% and more preferably 5 to 10% of the stoichiometric oxygen required for the combustion of said hydrocarbonaceous fuel fed to said burner.
As used herein, xe2x80x9cstoichiometric ratioxe2x80x9d means the ratio of oxygen fed, to the total amount of oxygen that would be necessary to convert fully all carbon, sulfur and hydrogen present in the substances comprising the feed to carbon dioxide, sulfur dioxide, and water.
As used herein, xe2x80x9cNOxxe2x80x9d means oxides of nitrogen such as but not limited to NO, NO2, NO3, N2O, N2O3, N2O4, N3O4, and mixtures thereof.
As used herein, xe2x80x9cfurnacexe2x80x9d means a device which, together with the burners and the means for feeding fuel and air as described herein, comprises a combustion chamber wherein said fuel combusts with said air to generate heat of combustion and gaseous combustion products, flue means for enabling said combustion products to leave the combustion chamber, and heating means for using said heat of combustion to produce steam.
As used herein, xe2x80x9cburnerxe2x80x9d means a means for feeding fuel and oxidant into a furnace either already commingled or such that the fuel and an associated stream of oxidant commingle within the furnace, whereby the fuel and oxidant combust. Examples of burners include burners as depicted in FIGS. 1 and 2, wherein a fuel stream and an oxidant stream are fed such that one stream surrounds the other as they enter the furnace, and burners depicted in FIG. 3 wherein fuel and oxidant enter the furnace from adjacent ports such that the fuel and oxidant commingle and combust inside the furnace.
As used herein, xe2x80x9cstaged combustionxe2x80x9d means combustion in a furnace wherein a portion of the combustion air (the xe2x80x9cover fire airxe2x80x9d) required for complete combustion of the fuel is fed to the furnace not through or immediately adjacent any burner but instead through one or more inlets situated between the burner(s) and the furnace flue means, and is fed without an associated feed of fuel.